In the past, methods for achieving smaller feature sizes have been to select a lithographic radiation source having a shorter wavelength, increase the numerical aperture (NA) of the lithographic system's lens or a combination thereof. While these methods have met with success, for each reduction in wavelength and/or increase in NA, the problems associated with taking advantage of such changes have been increasingly difficult to overcome.
Recently it has been suggested that rather than selecting a new lithographic radiation source with a shorter wavelength, e.g. 157 nm, the resolution of the current 193 nm standard source could be extended by the use of an immersion lithographic process. Such immersion lithographic processes replace the usual “air gap” between a lithographic tool's final lens and the substrate being exposed with a liquid, for example, water. The water, having a refractive index that is much greater than that of air, allows for the use of higher numerical aperture (NA) lens than would otherwise be possible while maintaining and acceptable depth of focus (DOF). Thus, it is believed that minimum feature sizes of 45 nm or less can be achieved with such an approach.
However, the successful implementation of immersion lithography for microelectronic device fabrication presents new problems that need to be resolved. For example, typically the substrate being exposed during a microlithographic process is repeatedly repositioned with respect to the lithographic tools lens at a high speed to achieve complete exposure of all portions of the substrate in a timely manner. With the addition of the aforementioned liquid (also referred to herein as an “immersion fluid”, “immersion medium”, or “IM”) residues of such a fluid that result from the repositioning have been observed and are the likely cause of imaging defects. While such repositioning related defects might be reduced or even eliminated by reducing the speed of the repositioning, such a decrease in movement speed (scanability) would result in an unacceptable decrease in the number of substrates per hour that a lithographic tool can fully expose.
In addition to problems relating to IM residuals and scanability, the use of an IM also raises concerns with regard to problems that can result from such a fluid being in direct contact with the imaging or photoresist layer that can lead to a reduction in that layer's ability to provide the desired image. For example, such problems can include, among others: 1) leaching of small molecules such as photoacid generators (PAGs) and PAG photoproducts from the photoresist film into the IM and 2) absorption of the immersion medium, or components thereof, into the photoresist film.
One method that has been investigated for the elimination or reduction of these and other problems associated with immersion lithography is the use of an intervening layer disposed overlying the photoresist film for receiving the IM. Such an intervening layer also referred to as a “top-coat” or “protecting layer,” can thus prevent or greatly reduce any imaging problems that might result from the leaching of small molecules from the photoresist layer or the absorption of the IM into such layer. With regard to scanability, the use of a top-coat allows for the design of such a material to have the specific properties necessary to eliminate or greatly reduce the possibility of IM residuals with little or no reduction in the speed of a tool's speed of movement.
The material used for such a “top-coat” or “protecting layer” should serve to protect the photoresist layer from the immersion fluid during the immersion photolithographic process. Such material should also be readily removable before or during the development of an image on the underlying photoresist layer.